: Provide feature negotiation for minimum-checksum-coverage

[DCCP]: Provide feature negotiation for minimum-checksum-coverage

This provides feature negotiation for server minimum checksum coverage which so far has been missing.

Since sender/receiver coverage values range only from 0...15, their type has also been reduced in size from u16 to u4.

Feature-negotiation options are now generated for both sender and receiver coverage, i.e. when the peer has `forgotten' to enable partial coverage then feature negotiation will automatically enable (negotiate) the partial coverage value for this connection.

---
 include/linux/dccp.h |    4 +--
 net/dccp/proto.c     |   52 ++++++++++++++++++++++++++++++++++++++-------------
 2 files changed, 41 insertions(+), 15 deletions(-)

@@ -545,8 +545,8 @@ struct dccp_sock {
 	struct dccp_ts_echo		*dccps_tstamp;
 	__u16				dccps_l_ack_ratio;
 	__u16				dccps_r_ack_ratio;
-	__u16				dccps_pcslen;
-	__u16				dccps_pcrlen;
+	__u8				dccps_pcslen:4;
+	__u8				dccps_pcrlen:4;
 	unsigned long			dccps_ndp_count;
 	__u32				dccps_mss_cache;
 	unsigned long			dccps_rate_last;